The liquid crystal does not emit by itself, and a light source is required to use the liquid crystal as a display device. A liquid crystal display device is comprised of a liquid crystal panel formed of a liquid crystal, orientation plates, electrodes, polarizers, etc. and an apparatus for applying light to the panel such as a lighting device generally referred to as a backlight and the like, and uses a reflecting sheet to reflect the light of the lamp toward the screen efficiently or the like.
The backlights of the liquid crystal display device are generally classified into two types broadly i.e. the edge light type backlight and direct type backlight.
The edge light type backlight is a backlight usually used in small-size liquid crystal display devices used in cellular telephones, personal digital assistants, etc. The edge light type backlight is often comprised of a light source such as an LED, cold-cathode tube, etc. a light guide plate made of a transparent resin such as acrylic resins, etc. formed in the shape of a wedge, and a reflecting sheet disposed on the side face of the light guide plate on the opposite side to the liquid crystal panel. In the edge light type backlight, the light from the light source such as an LED, cold-cathode tube, etc. disposed on the end face of the light guide plate is applied from the end face of the light guide plate. The light incident to the light guide plate comes out of the light guide plate from the side face of the light guide plate in the process of passing through the guide light plate. The light transmitted from the side face on the liquid crystal panel side of the light guide plate lights the liquid crystal panel, but the light transmitted from the side face on the opposite side to the liquid crystal panel side of the light guide plate is not able to light the liquid crystal panel. Therefore, a reflecting sheet for reflecting the light is installed on the side face on the opposite side to the liquid crystal panel of the light guide plate to reflect the light coming out of the side face of the light guide plate onto the liquid crystal panel side, and thus using the reflecting sheet is usually performed to irradiate the liquid crystal panel effectively with the light from the light source.
The direct type backlight is a backlight where a plurality of light source lamps such as cold-cathode tubes, etc. is installed on the opposite side to the display surface of the liquid crystal panel, and is used in large screen liquid crystal display devices used in large-size televisions and the like. In the large screen liquid crystal display device, since the edge light type backlight is limited in intensity of the light source lamp in increasing the brightness to the satisfied level, the direct type backlight using a plurality of light source lamps is usually used. The light of the light source lamp is emitted to the opposite side to the liquid crystal side, and therefore, in the direct type backlight, it is usually implemented to provide the reflecting sheet on the opposite side to the light crystal side of the light source lamp so as to irradiate the liquid crystal panel effectively with the light of the light source.
Recently, moving pictures have often been displayed in personal computers as well as televisions, and the liquid crystal display device is required to be brighter. Therefore, in the backlight used in the liquid crystal display device, reflecting sheets with a reflectance of 90% or more are used in many cases. To increase the brightness of the liquid crystal display device, the output of the light source such as the cold-cathode tube tends to increase, and there is a tendency that the temperature of the backlight being used is higher. Therefore, the resin used in the reflecting sheet is required to have heat resistance of 80° C. approximately close to the heat resistance temperature of the liquid crystal material. The reflecting sheet used in the backlight of the liquid crystal display device is thus required to be a reflecting sheet of a resin composition that is easy to form into a sheet and that is excellent also in heat resistance. Further, in the backlight used in the large screen liquid crystal display device such as the large-size television, the reflecting sheet with a large area is exposed to strong light over the long term. Therefore, the reflecting sheet is required to be low in discoloration and degradation due to the light of the light source, and is hard to cause deformation such as curling, etc. due to increases in temperature and moisture over long periods.
In sheets of resins containing pores and/or voids inside thereof, when the light is applied, it has been known well that the sheets look white by the light being reflected, or show pearl-like shines. The reason why the resin containing pores and/or voids inside thereof reflects the light well is considered as described below. The refractive index of the resin ranges approximately from 1.4 to 1.6, the refractive index of air is about 1, and the reflectance caused by the difference in the refractive index between the resin and air is only about 4%. However, in the sheet of a resin containing many pores and/or voids inside thereof, since many interfaces exist between the resin and air inside the sheet, the light applied to the sheet is reflected many time inside the sheet. As a result, in the sheet of a resin containing many pores and/or voids inside thereof, the applied light is reflected in the most part inside the sheet, and as a result, it is considered that the reflectance increases as the entire sheet.
Further, since many pores and voids contained inside the resin are usually different in shape and size from one another, the light reflected in the interface of the pores and voids is seldom reflected collectively in one direction, and the directions of reflected light vary with the pores and voids. Therefore, the reflection of light applied to the resin sheet containing many pores and voids inside thereof tends to be diffuse reflection where the incident light beams are reflected in every directions. As a sheet of a resin composition containing pores and voids inside thereof, there have been known (a) sheets obtained by stretching the resin with an inorganic powder added thereto, cleaving the interface between the resin and the inorganic powder, and forming pores inside the resin, and (b) sheets obtained by dissolving pressurized inert gas in the resin, reducing the pressure to cause foaming, and forming voids inside the resin.
As the resin sheet of (a), for example, Patent Document 1 discloses a white sheet of polyethylene terephthalate with a void rate of from 7% to 30% formed by melting, extruding and two-way stretching a polyethylene terephthalate resin containing 5 percent to 30 percent by weight of fine-particle calcium carbonate. Such a reflecting sheet contains the inorganic powder such as calcium carbonate and barium sulfate with a high ratio of several tens of percent by volume to the entire reflecting sheet. Therefore, although the void rate of from 7% to 30%, the reflecting sheet has a high density of 0.8 g/cm3 or more, and is required to be light weight. Further, Patent Document 2 discloses a white polyolefin film containing calcium carbonate in the inner layer portion and surface layer portion. In such a reflecting sheet, since a large amount of inorganic powder is used such as calcium carbonate, the density is 0.7 g/cm3 or more and thus high, the refractive index of calcium carbonate in the surface layer portion is low, and excellent reflective performance is not obtained.
As the resin sheet of (b), for example, Patent Document 3 discloses an optical reflecting sheet containing fine voids inside thereof formed by dissolving an inert gas such as a carbon dioxide gas in thermoplastic polyester under pressurized atmosphere, and then, heating at normal pressure for foaming. This reflecting sheet is made of resin without containing an inorganic powder, but the manufacturing processes of the sheet tend to be large-scale complicated processes, because it is necessary to keep the resin sheet in an atmosphere of pressurized inert gas for a predetermined time so as to dissolve the inert gas in the resin sheet, and processes of heating and foaming are required after removing the resin sheet with the inert gas dissolved therein and restoring to the normal pressure. Further, in Patent Document 3, voids are formed by fine foaming of polyester. The voids are fine foam but have void diameters ranging from 5 μm to 10 μm at minimum. It is required to contain many fine voids to obtain high reflective performance. Therefore, the sheet thickness ranges from 0.8 mm to 1.2 mm and is thus thick. Thinner sheets with excellent reflective performance are desired for crystal liquid display.
In contrast to diffusion reflection, such reflection is referred to as regular reflection where the angle of light incident on the reflecting surface and the reflection angle are symmetry, and the reflecting surface has a mirror-shaped surface. As a resin sheet causing regular reflection, there is known (c) polyester resin sheets obtained by coating the sheet surface with silver or the like with a high reflectance by a method of deposition, etc.
In the reflecting sheet of (c), metal particles such as silver, etc. to coat the surface of the resin sheet are tend to coagulate by heat of the light source lamp being used or the like, and to be oxidized by trace acidic gas component contained in air, and it is known that the sheets cause discoloration of metal such as silver, etc. and deterioration of the reflectance. Therefore, it is carried out that the particles of metal such as silver, etc. applied onto the sheet surface are coated with a resin to prevent contact with air, the processes are required to apply light coatings of metal and resin to the surface of the resin sheet several times, and the manufacturing processes of the sheet tend to be large-scale complicated processes. The reflecting sheet of this example is a mirror-shaped sheet for causing regular reflection, and in the direct type backlight used in the large screen liquid crystal display device of the large-size television, it is known that light beams of a plurality of arranged light source lamps interfere with one another and are apt to cause uneven lightness in the liquid crystal screen. Therefore, the direct type backlight usually uses the reflecting sheet for causing diffusion reflection.
Although the reflecting sheets of (a) and (b) as described above are the sheets for causing diffusion reflection, (a) has the problem of the heavy weight due to the use of a large amount of inorganic particles in the sheet as described above, and (b) has the problem that the sheet thickness is 0.8 mm to 1.2 mm and thus high because the void diameters are large. Desired are reflecting sheets of diffusion reflection with such problems solved.
Further, Patent Document 4 discloses a reflecting sheet made of a resin composition containing 50 or more percent by volume to less than 80 percent by volume of a polypropylene resin, and 20 or more percent by volume to less than 50 percent by volume of a resin that causes phase separation from the polypropylene resin at temperatures enabling stretching of the polypropylene resin. This Document provides the reflecting sheet which has a high reflectance of 90% or more without containing inorganic particles using a normal simplified resin stretching manufacturing apparatus. However, in the manufacturing method for monolayer extrusion film forming or the like, there are cases that anisotropy occurs in the total reflection index by light incident direction, and there is the case of requiring control of the reflecting sheet direction in handling.
Meanwhile, the conventional reflecting sheet is generally used in the shape of a flat plate, but for the purpose of increasing reflection efficiency, often undergoes mechanical folding processing or cut-folding processing or is processed in two-dimensional curved form to use. Moreover, for the purpose of further enhancing reflective performance, it is also proposed to process the reflecting sheet in optimal form in accordance with the usage mode of the backlight. For example, in the LED direct type backlight scheme, to enhance reflection efficiency, proposed is a reflecting sheet processed to be folded in concave form immediately below LEDs in accordance with the arrangement of a high number of LEDs arranged in the shape of a lattice pattern. When the reflection efficiency is improved and the lamp images of LEDs, etc. are apt to disappear by devising the shape and controlling the reflection direction, it is possible to shorten the distance between the lamp and diffusion plate, and it is also possible to form thinner backlight units. Patent Document 6 discloses a lighting device provided with a light emitting element having a concave-shaped reflecting layer. Further, Patent Document 7 discloses a concave-shaped reflecting sheet where a plurality of reflecting surface portions is disposed adjacent to one another, and surface sides between adjacent reflecting surface portions are continuous in the ridge line. In the LED backlight scheme, the diffusion plate is usually disposed on the three-dimensional processed concave-shaped reflecting sheet. When the diffusion plate is directly disposed on the reflecting sheet, in the reflecting sheets proposed in Patent Documents 6 and 7, the ridge line portion where the reflecting surfaces on the upper surface of the reflecting sheet are adjacent to one another is directly in contact with the diffusion plate, produces shade, and becomes a cause of fluctuations in brightness or the occurrence of lamp image. Therefore, measures are required to provide a suitable clearance between the reflecting sheet and diffusion plate by providing the diffusion plate with an appropriate support member.
Meanwhile, the edge light type backlight is used in small-size liquid crystal display devices such as mobile type devices, and is particularly required to thin the display device itself, and the three-dimensional processed shape in the entire sheet increases the thickness of the device and is not desired. Accordingly, a method is proposed of providing the sheet surface with a fine three-dimensional structure surface, and thereby controlling the reflection direction. For example, Patent Document 5 proposes an optical sheet where the light output in the slanting direction from the back of a light guide plate is reflected off the optical sheet with a metal thin film formed on the surface thereof having a concavo-convex pattern, and is applied again perpendicularly to the back of the light guide plate. In order to perform fine shaping to control the reflection direction, for example, there is a method of using a UV curable resin and curing the resin on the substrate surface by UV to provide the shape. However, such a method is complicated and expensive, and further has the problem of adhesion to the substrate surface. As an easy and inexpensive method, there is a press compression forming using a shaping die or a thermal compression forming for performing roll compression forming using a roll with a shaping die. However, the conventional reflecting sheet obtained by performing two-way stretching on a sheet made of inorganic material and polypropylene has large thermal shrinkage at thermoforming temperatures and is hard to obtain formed components in desired form. Further, there are problems that the reflective performance degrades by the sheet being thinner due to compression forming, or by formed pores and/or voids being crushed due to stretching, and the like. Furthermore, the reflecting sheets obtained by biaxial stretching of polyethylene terephthalate or fine foaming of the resin have problems that the resin deforms significantly due to properties of the polyethylene terephthalate resin when the temperature is increased to the softening temperature, and thus are hard to undergo surface fine processing by compression forming.    Patent Document 1: Japanese Laid-Open Patent Publication No. H06-89160    Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-004195    Patent Document 3: Japanese Patent Gazette No. 2925745    Patent Document 4: International Publication No. 2005/096036 Pamphlet    Patent Document 5: Japanese Laid-Open Patent Publication No. 2001-338505    Patent Document 6: Japanese Laid-Open Patent Publication No. 2004-185972    Patent Document 7: Japanese Patent Gazette No. 3928395